Nasa scientists are trying to determine whether the US space shuttle Discovery will need more repairs before being given the all clear to return to Earth.

On Wednesday an astronaut carried out a pioneering spacewalk to the orbiter's underside to remove protruding material threatening the heatshield's integrity.

But engineers now fear the thermal blanket near the cockpit is damaged and needs repairs to stop it tearing off.

It is the first mission since Columbia overheated and broke up on re-entry.

High speed impact

We're just pounding this flat. We're not going to leave any stone unturned at this stage, to make sure the crew's safe during entry

Deputy shuttle program manager Wayne Hale

Nasa is currently analysing a video of an area underneath the cockpit window to see if it suffered damage during lift-off.

Initial photographs appeared to indicate that the thermal blanket below the cockpit window had been punctured at one end.

The thermal blanket is made up of a quilt-like, padded fabric and serves as insulation from the intense heat generated during re-entry to Earth's atmosphere.

Although not heavy, there is concern that a section measuring about 30cm (one foot) could tear off as the shuttle tries to re-enter Earth's atmosphere and hit the main body of the orbiter at such high speed that it could cause a grave injury.

Increased safety measures

Engineers have been working overnight in a wind tunnel in an attempt to replicate the conditions around Discovery to see if further damage is likely.

"We want to know if it can come apart on re-entry and if so where the pieces might go and what they may hit, so a number of wind tunnel tests at various Nasa centres have been going on over the night from Kennedy Space Centre in Florida to Ames Research Centre in California, and later this morning our mission managers will determine whether we need to do anything about that," he added.

If the test results prove unfavourable another spacewalk could be scheduled for Friday to try to trim the blanket.

"It's not so much a thermal concern to us as much as it is a debris concern, Nasa spokesman William Jeffs told the BBC.

Crew unfazed

During an earlier spacewalk crew members tested repair techniques for the heatshield tiles amid heightened fears that there could be another overheating incident like that which caused Columbia to break up on re-entry in 2003, killing all on board.

"I think in the old days, we would not have worried about this nearly so much," deputy shuttle program manager Wayne Hale said of this latest concern. "We're just pounding this flat. We're not going to leave any stone unturned at this stage, to make sure the crew's safe during entry."

The crew are wearing special shirts bearing the Columbia mission patch

Discovery and its crew of seven astronauts is due to return to Earth on 8 August.

Despite the latest tests the crew say they are not worried about their re-entry prospects.

"We are not too concerned about it, we think its going to be fine," shuttle commander Eileen Collins said.

Astronaut Stephen Robinson, who carried out the repairs to the shuttle's heatshield, said the mission was in fact a test flight: "We been dealt problems and we have dealt with those problems, we know much more about our ship than crews on any other mission and it is actually a much more positive experience than is being portrayed."

Later on Thursday the Discovery crew will pay tribute to their colleagues killed during the Columbia disaster, as well as previous astronauts and Russian cosmonauts who have died in the pursuit of space exploration.